Ink jet recording is a printing method in which droplets of ink are sprayed and made to adhere to a recording medium such as paper. The ink used for this purpose is generally produced by dissolving any of various water-soluble dyes in water alone or in both water and a water-soluble organic solvent. The images formed by such inks containing water-soluble dyes are generally held to have poor resistance to water and light.
In contrast, an ink obtained by dispersing a pigment in an aqueous medium has excellent resistance to water and light. For instance, Japanese Laid-Open Patent Applications S64-6074 and S64-31881 propose aqueous pigment inks in which carbon black is dispersed with a surfactant or a polymer dispersant (Patent Documents 1 and 2). With these inks, however, if the colorant content in the ink is increased in order to raise the print density of the recorded matter, this is sometimes accompanied by a sharp increase in ink viscosity as well. Also, an excessively large amount of surfactant or polymer dispersant must be used for the carbon black to be stably dispersed in the ink, which can sometimes lead to a decrease in print stability due to foaming or a decrease in the anti-foaming property of the ink.
To solve these problems, Japanese Laid-Open Patent Applications H8-3498 and H10-120958 have disclosed a self-dispersing carbon black dispersion in which at least a specific amount of surface-activated hydrogen or a salt thereof is introduced to the carbon black surface so that just the carbon black can be dispersed in an aqueous solvent without having to use a surfactant, polymer dispersant, or the like (Patent Documents 3 and 4). Japanese Laid-Open Patent Application H10-110127 discloses a method in which sulfonic acid groups are introduced to the carbon black surface (Patent Document 5). Further, an ink jet ink containing the above-mentioned surface-modified carbon black and a glycol ether has been proposed in Japanese Laid-Open Patent Application H10-95941 (Patent Document 6).
A variety of these so-called self-dispersing pigments that need no dispersant have been developed, each with its own advantages, such as those with which the OD (optical density) value increases when these pigments are used as colorants in inks, or which are easy to handle because the viscosity of the ink has been adjusted to a suitable range, or which do not require that miscibility with dispersants or various added solvents be taken into account. These pigments are generally anionic surface-modified types in which carbonyl groups, carboxyl groups, hydroxyl groups, sulfone groups, ammonium groups, or other such hydrophilic functional groups are bonded, either directly or via alkyl groups, aryl groups, or the like, to the surface of the pigment.
Unfortunately, with an ink jet recording ink composition in which a self-dispersing pigment such as these is used, the OD value is high, but the fixability of the ink to the recording medium (and particularly the fixability of the pigment in the ink) is inadequate, which is a problem in that it results in poor smear resistance.
Also, gloss has in recent years come to be required of images formed on special paper or other such recording media by an ink jet recording ink composition.
Patent Document 1: Japanese Laid-Open Patent Application S64-6074
Patent Document 2: Japanese Laid-Open Patent Application S64-31881
Patent Document 3: Japanese Laid-Open Patent Application H8-3498
Patent Document 4: Japanese Laid-Open Patent Application H10-120958
Patent Document 5: Japanese Laid-Open Patent Application H10-110127
Patent Document 6: Japanese Laid-Open Patent Application H10-95941
It is an object of the present invention to provide an ink jet recording ink composition with good reliability and print quality, a high OD value, and good fixability that affords excellent smear resistance. It is a further object of the present invention to provide an ink jet recording ink composition that can impart gloss.